Hitherto, as a soft segment for use in a polyurethane and a urethane-, ester- or amide-based thermoplastic elastomer, a polyester polyol and/or a polyether polyol, of which polymer terminal is a hydroxyl group, have been employed (e.g., see Patent Documents 1 and 2).
Of these, since a polyester polyol represented by a polyadipate polyol is poor in hydrolysis resistance, for example, the use of a polyurethane using the same is considerably limited since cracks may occur and mold may grow on the surface thereof within a relatively short period of time. On the other hand, a polyurethane using a polyether polyol has good hydrolysis resistance but the polyurethane has a disadvantage that it is poor in weather resistance and resistance to oxidative degradation. These disadvantages are, respectively, attributed to the presence of ester groups and ether groups in the polymer chains. With regard to a polyester- or polyamide-based thermoplastic elastomer, recently, highly enhanced thermal resistance, weather resistance, hydrolysis resistance, mold resistance, oil resistance, and the like have been demanded and hence the elastomer has problems similar to the case of the polyurethane.
In order to solve these problems, as a polyol component, a polycarbonate polyol, specifically a polycarbonate polyol of 1,6-hexanediol has been used since a carbonate bond in a polymer chain is extremely stable.
However, the polycarbonate polyol of 1,6-hexanediol is crystalline and hence is solid at normal temperature, so that there is a problem that handling thereof is difficult.
Moreover, in the case of using the polycarbonate polyol of 1,6-hexanediol as a soft segment of a polyurethane, the polyurethane has a disadvantage that flexibility and low-temperature properties are poor although hydrolysis resistance, weather resistance, resistance to oxidative degradation, heat resistance, and the like are improved. Namely, owing to large tendency to crystallize, the polyurethane using the polycarbonate polyol of 1,6-hexanediol has a problem that the soft segment component tends to cause crystallization hardening to impair elasticity and, in particular, recovery of elasticity at low temperature is remarkably lowered. Furthermore, the oil resistance is improved as compared with the polyether polyol but is still insufficient.
In order to solve these problems, various methods have been proposed.
For example, there has been disclosed an aliphatic copolycarbonate diol using 1,5-pentanediol and 1,6-hexanediol (e.g., see Patent Document 3). In this method, the resulting polycarbonate diol has, between carbonate bonds, a portion wherein an odd number of methylene groups are present. Thereby, structural regularity of the polycarbonate diol molecule is inhibited, crystallinity decreases, and further it becomes amorphous. However, even when the technology is applied, owing to its high viscosity, it is still insufficient in view of handling ability depending on the method employed.
Furthermore, there have been proposed thermoplastic polyurethanes which are produced using, as a soft segment, a copolymerized polycarbonate diol obtained from 1,6-hexanediol and 1,4-butanediol or 1,5-pentanediol (e.g., see Patent Documents 4 and 5). These thermoplastic polyurethanes are also remarkably excellent in flexibility in addition to the above properties of the thermoplastic polyurethane produced using as a soft segment the polycarbonate diol obtained from 1,6-hexanediol alone, and hence has attracted attention recently. However, as a result of investigation of the present inventors, the above thermoplastic polyurethanes produced using as a soft segment the copolymerized polycarbonate diol obtained from 1,6-hexanediol and 1,4-butanediol or 1,5-pentanediol have problems that a physical property balance between oil resistance and flexibility is insufficient and thus uses thereof are limited.
In addition, an aliphatic carbonate diol using 3-methyl-1,5-pentanediol is disclosed (e.g., see Patent Document 6). By introducing the structure having such a side chain, the structural regularity of the polycarbonate diol is disturbed and crystallinity is lowered. However, as a result of investigation of the present inventors, a thermoplastic polyurethane produced using as a soft segment the above polycarbonate diol obtained from 3-methyl-1,5-pentanediol has a problem that oil resistance is insufficient although an improvement in flexibility is observed.
As copolymerized polycarbonate diols other than the above ones, there have been disclosed one using 1,6-hexanediol and trimethyl-1,6-hexanediol (see Patent Document 7) and one using 1,9-nonanedol and 2-methyl-1,8-octanediol (see Patent Document 8). When they are converted into thermoplastic polyurethanes, there are problems that oil resistance and flexibility are still insufficient in the former case and oil resistance is insufficient although flexibility is improved in the latter case. Moreover, there also a problem that the diols as raw materials are not easily available in both cases.
The polycarbonate diol of the invention contains 5-methyl-1,3-dioxane-2-one as an impurity. In the case that the amount of the impurity is large, as a result of investigation of the present inventors, it has been found that hydrolysis resistance is particularly lowered when the diol is converted into a thermoplastic polyurethane. On the other hand, there has been disclosed a process for producing a polytrimethylene carbonate diol having a similar structure (Patent Document 9). In Production Examples thereof, it is disclosed that 3% or less of 1,3-dioxan-2-one is contained as an impurity but the impurity does not affect weather resistance and hydrolysis resistance when the diol is converted into a thermoplastic polyurethane even in the case where the amount is large, unlike the impurity of the invention, and only exerts an influence of lowered yield of the polycarbonate diol.    Patent Document 1: U.S. Pat. No. 4,362,825    Patent Document 2: U.S. Pat. No. 4,129,715    Patent Document 3: JP-B-5-29648    Patent Document 4: JP-A-5-51428    Patent Document 5: JP-B-7-684    Patent Document 6: JP-B-4-1764    Patent Document 7: JP-A-2-49025    Patent Document 8: JP-A-5-339816    Patent Document 9: JP-A-2004-35636